Usually, projector is used in cooperation with writing on a whiteboard for clearly presentation. Therefore, when the whiteboard is used, the cooperating projector must be blanked to avoid the projecting light from interfering with the illustration shown on the whiteboard. Although “blanking” is used for addressing the foregoing operation, the operation of blanking is performed in a digital projector by using optical chassis to “divert” the projecting light from the intended projecting direction. Usually, the blanked projecting light is reflected and diverted toward the upper lid or the lower lid of the optical chassis, such that the projecting light won't be projected out from the lens of the projector. However, since the projecting lens is totally blocked out while blanking the projector of prior arts, the user is unable to determine whether the projector is under operational mode and will start the projector again. The situation not only is inconvenient for operation, but also will cause malfunction of the projector.